The present invention relates to the general field of reconfigurable logic cells comprising a plurality of inputs and capable of performing a certain number of logic functions with which logic signals provided on their inputs may be processed.
More specifically, the invention proposes reconfigurable logic cells with reduced production costs.
The production costs of integrated circuits is essentially due to the cost of the design of the masks useful for lithography and to the cost of automatic equipment used for producing integrated circuits, notably photolithography devices.
These production costs regularly increase with the present trends towards miniaturization and complexity of the architectures. Also, more and more sophisticated devices are required in order to meet a certain number of problems generated by miniaturization which generates problems of leakage currents and behavior reliability. All this considerably complexifies the design of the masks.
Dedicated circuits, for example ASIC (Application Specific Integrated Circuit) circuits which exist since the nineteen seventies, have very high integration density and are designed for a specific application. Their design is long and costly and the slightest design error requires a new revision of the circuit.
In order to guarantee larger flexibility, programmable circuits have been developed. They provide good tolerance to errors and allow reuses for different applications. In particular, the masks may be reused in various contexts. Different types of architecture exist for such circuits, for example FPGA (Field Programmable Gate Array), CPLD (Complex Programmable Logic Device) or further PLA (Programmable Logic Array) circuits. They appeared in the nineteen eighties (EP-A-0 264 334, FAIRCHILD SEMICONDUCTOR [US], Apr. 20th 1988).
Their less complex flexibility and design than those of dedicated circuits such as ASICs, have made these circuits popular in the industrial environment. They are often known as “universal functions”. Such programmable circuits use complementary static CMOS logic. With these circuits, it is possible to connect together basic components which are the basic logic functions.
It is thus possible to produce any logic function desired by the user, but these programmable circuits remain nevertheless greatly disadvantaged in comparison with ASICs in terms of performance and consumption.
Indeed, the latter are generally optimized in order to operate at high speed and/or low consumption, which is not the case for programmable circuits which use interconnection networks for connecting the different basic components and are therefore handicapped by parasitic capacitances provided by these interconnections between the basic components.
Today there exist other logic styles for implementing the basic components and with which reconfigurable cells may be made. The implementation of such reconfigurable cells instead of conventional logic cells, gives the possibility of contemplating reduction of the interconnections and access to full use of the resources for the computation.
This is beneficial in terms of performance and consumption but also in terms of costs, since the silicon surface and the number of metal layers required for routing interconnections are reduced.
With these approaches it is possible to produce a certain number of logic functions and they use Hall effect devices or further CNTFET (Carbon Nanotube FET) devices, or further RTD (Resonant Tunneling Diode) components, in combination with an HFET (Heterostructure FET) component.
However, some of these manufacturing processes are not mature, notably CNTFET technology, and some are complex and therefore costly, notably RTD/HFET and CMOS/Hall effect technologies.
Indeed, components using the Hall effect cannot be easily integrated with CMOS technology, which complicates manufacturing and makes it costly.
RTD components as for them are not compatible with CMOS technology, which implies external connections and complexification of the final circuit.
CNTFET components are difficult to utilize from an industrial point of view since this technology is not yet sufficiently mature for leading to industrial production.
Nevertheless it is true that there exists a great need for reconfigurable circuits, easy to design and integrate, and inexpensive, with which, by their possibility of being used in multiple applications, it would be possible to make the cost of the masks profitable in the medium term.